1. Technical Field
The present invention pertains to improvements for envelopes containing a fill for use in electrodeless lamps and has particular, although not limited, utility in lamps of the type disclosed in U.S. Pat. No. 5,404,076 and PCT International Publication No. WO 92/08240, the disclosures of which are expressly incorporated by reference herein in their entirety.
2. Discussion of Related Art
Electrodeless lamps of the type with which the present invention is concerned are comprised of a light transmissive bulb having an envelope containing a plasma-forming medium. A bulb is an envelope usually mounted on an elongate, radially projecting supporting stem. A microwave or radio frequency (RF) energy source has its output energy coupled through the envelope via a coupling arrangement to excite a plasma, resulting in a light discharge. The energy coupling arrangement customarily includes a microwave cavity to which microwave energy is coupled, and the bulb is mounted inside the cavity. Alternatively, the energy may be coupled to the fill through an inductive arrangement (e.g. an excitation coil surrounding the bulb) or a capacitive arrangement (e.g. a bulb between two electrodes). Electrodeless lamps may include an internal reflector or may be reflectorless, the latter requiring a separate light reflector to direct light emissions. A separate reflector is not readily inserted within the cavity since the cavity customarily includes a first solid conductive structure at one end, usually a cylindrical wall, joined to a second cylindrical structure formed of a mesh (e.g., tungsten mesh), such that microwave energy is contained within the cavity but light is transmitted outwardly. A separate reflector customarily has an axis of symmetry approximately coincident with the axis of the cavity and surrounds the cavity. The surface of the reflector may follow a simple geometric contour such as an ellipsoid or paraboloid and may be comprised of a plurality of annular facets, each sized and oriented to direct reflected light in a desired direction. A bulb is located along the axis of the cavity within the mesh structure and includes an envelope portion and a stem. The stem may also be located along the axis of the cavity or may be positioned at an angle with respect to the axis. The stem may be fixed (e.g. fastened to the first solid structure) or may be secured to a motor shaft for rotation of the bulb. If the envelope is essentially spherical and the light source is energized by microwaves, the resulting light produced is emitted with significant power in all directions. However, only a portion of the solid angle about the envelope corresponds to the mesh and substantial light is blocked by solid structure and not received by the reflector. The blocked portion of the solid angle about the envelope corresponds to the solid structure of the cavity and the end wall of the cavity (e.g. the wall with the coupling slot) and therefore light directed toward the blocked portion is occluded and lost.
In order to more efficiently direct the light outwardly and away from the coupling wave guide structure, various techniques have been suggested. For example, U.S. Pat. No. 5,334,913 (Ury et al) discloses a supplemental non-conductive optical reflector located within the cavity. Although reflectors disposed apart from the envelope but within the cavity can be effective, they consume space and add to the cost of the overall lamp.
Another problem encountered using spherical glass envelope structures is that significant thermal stresses are created in the envelope wall. In particular, internal heat from the plasma necessitates use of cooling fans to control the temperature of the envelope wall. In prior art lamps, rotation of the bulb about its support stem axis is commonly done for a number of reasons, one of which is to evenly distribute flow of cooling air over the envelope wall. Use of a separate non-conductive internal optical reflector has, therefore, presented additional problems in that special conduits for jets of cooling air must be routed around the internal reflector.